In medical contexts, there are needs for a sensor (catheter) which is suitable for insertion in elongate and generally duct-like body cavities of varying cross-section, e.g. in the urinary tract in order to indicate changes in the cross-section of the cavity. In such duct-like cavities, the tissue pressure may, e.g. through muscular action, vary considerably between different regions of the cavity. There is, therefore, a need to be able, in addition to generally indicating changes in the cross-section of the cavity, also to be able to indicate locally occurring changes in the cross-section.
Patent specification U.S. Pat. No. 4,711,249 discloses a tubular sensor disposed to be inserted in an elongate cavity and detect changes in the dimensions of the cavity. The sensor consists of a number of sequentially disposed and spaced apart rigid tube sections in whose ends are inserted mechanically rigid bodies which sealingly connect to the surrounding tube sections. Between the ends of the tube sections, there are formed circumferentially extending grooves which are covered by elastic membranes or diaphragms. Between the membranes and the rigid bodies, spaces are formed.
Each one of the rigid bodies has a central chamber which, through an aperture, is connected to the space between the rigid body and the membrane. In its one end, the chamber has a mechanically rigid closure and in its other end a flexible closure. The space between the membrane and the rigid body as well as the chamber in the body is filled with liquid. Pressure changes in the chamber of the body are registered by a sensor abutting against the flexible closure.
The apparatus disclosed in U.S. Pat. No. 4,711,249 suffers from the drawback that only in those parts of the cylindrical sensor where the elastic membranes bridge the space which is formed between the ends of the tubular portions can the sensor register a change of the circumference of a surrounding organ. This implies that only short portions of the cylindrical sensor are capable of registering changes in the circumference of the surrounding organ. In regions which are located between the membranes, no detection of changes in the circumference of the surrounding organ takes place, in addition to which the sensor prevents the surrounding organ from assuming a circumference which is less than the circumference of the sensor.